In recent years, thin-film type solar cells produced by using a chalcopyrite compound semiconductor have been employed, the thin-film type solar cells produced by using the compound semiconductor have a basic structure in which an Mo electrode layer that is used as a positive electrode is formed on a soda-lime glass substrate, a light absorbing layer consisting of a CIGS film is formed on the Mo electrode layer, a buffer layer consisting of zinc sulfide (ZnS), cadmium sulfide (CdS), and the like is formed on the light absorbing layer, and a transparent electrode layer that is used as a negative electrode is formed on the buffer layer.
As a method for forming the light absorbing layer described above, a vapor deposition film-forming method is known. Although a light absorbing layer obtained by the method may exhibit high energy conversion efficiency, the vapor deposition film-forming method attains slow speed for forming a film. Hence, when a film is formed on a substrate having a large area, the uniformity of the thickness distribution of the film thickness is readily reduced. Thus, a sputtering method for forming a light absorbing layer has been proposed.
As a method for forming the light absorbing layer described above, a method (selenization method) has been proposed in which an In film is formed by sputtering using an In target in the beginning, a Cu—Ga binary-based alloy film is formed on the In film by sputtering using a Cu—Ga binary-based alloy target, and a stacked precursor film consisting of the obtained In film and Cu—Ga binary-based alloy film is subjected to heat treatment in a Selenium atmosphere and thereby a CIGS film is formed.
In order to improve an electric generation efficiency of the light absorbing layer formed of the CIGS film, for example as shown in Non-Patent document 1, an addition of Na into the light absorbing layer by the diffusion from an alkaline glass substrate is effective. However, there is an inconvenience in that in a case of a flexible CIGS solar battery in which a polymer film or the like instead of the alkaline glass substrate is a base material, a supply source of Na is lost because there is no alkaline glass substrate.
Accordingly, it has been proposed that a lift-off layer by sodium chloride (NaCl) is provided and Na is diffused into the light absorbing layer from the lift-off layer in order to improve photoelectric conversion properties of the flexible CIGS solar battery formed on the polymer film (for example, refer to Patent Document 1).
With respect to the addition of Na, a method has been proposed in which a soda-lime glass is formed between a Mo electrode layer and a substrate (for example, refer to Non-Patent Documents 1 and 2). However, when the soda-lime glass is deposited by the proposed method, the producing processes increase and the productivity is reduced.
There is a proposal that fluoride sodium (NaF) is added into a Mo electrode layer and Na is diffused into the light absorbing layer from the Mo electrode layer (for example, refer to Patent Document 2). However, when NaF is added into the Mo electrode layer, a large amount of Na is concentrated between the Mo electrode layer and a substrate, and there is a possibility of the occurrence of peeling off at an interface between the Mo electrode layer and the substrate.
Also, since the amount of diffusion of Na from the glass substrate or the like is difficult to control, as a method for adding Na regardless of diffusion, a method that a specific Na compound is directly added into the light absorbing layer has been proposed. For example, it is proposed that a diffusion blocking layer is provided between the glass substrate and the absorbing layer and the diffusion of Na from the glass substrate is blocked, and sulfide sodium (Na2S) and selenide sodium (Na2Se) are added into the light absorbing layer and the content of Na with respect to the light absorbing layer is controlled (for example, refer to Patent Document 3).
In order to add a specific Na compound directly into the light absorbing layer, a Cu—Ga sputtering target containing Na of 0.05 to 1 at. % and containing Na in a state of NaF compound has been proposed (for example, refer to Patent Document 4).
Here, for example in Non-Patent Document 3, a CIGS film in which the addition of Na is 2% is disclosed, and it is known that a CIGS solar battery having a good electrical efficiency can be obtained with a high concentration of Na addition that is not studied conventionally.
Also, fluoride sodium (NaF), chloride sodium (NaCl), sulfide sodium (NaS), selenide sodium (Na2Se), sodium sulfate (Na2SO4) and sodium sulfite (Na2SO3), selenium sodium (Na2SeO4) and sodium selenite (Na2SeO3), or the like, is proposed as Na compounds that is added in a sputtering target consisting of copper, indium, and gallium (for example, refer to Patent Documents 5 and 6).